Because the field oxide film formed by means of the LOCOS method used in the past in the interelement separation of semiconductor devices such as an LSI has problems such as bird's beak, it is difficult to respond to the downsizing of the field regions accompanying the change to microscopic dimensions and high integration of the elements in recent years. Thus, even though the field region is reduced in size, as an element separating method that conducts the interelement separation with good reliability, the trench separator is being used. This trench separator is one wherein the interelement separation is conducted by imbedding an insulating film within a groove (trench) formed in the element separating region (field region) of the semiconductor element.
FIG. 16 shows a structure used in the past wherein a trench separator is applied to the substrate of the so-called SOI structure wherein the elements are formed in a silicon semiconductor layer provided on top of an insulating layer.
The substrate 1 has an SOI structure wherein the substrate main body section 2 and the substrate surface section 3 are mutually electrically separated and isolated by means of the silicon oxide layer 4. For this type of SOI substrate, the SIMOX substrate or the like is well known, wherein, after two monocrystalline silicon substrates are glued together with an oxide film membrane interposed, the glued substrate is made thin by grinding the substrate on the element forming surface side, and an embedded oxide membrane is formed by ion injection of oxygen at a prescribed position and depth of the monocrystalline silicon substrate.
As is illustrated, the groove (trench) 5 is formed in the element separating region of the substrate surface section 3, and a trench-like element separating structure is formed by means of the silicon oxide film 6 provided on the side walls of this groove 5 and the polycrystalline silicon 7 filled inside the groove 5 on the inner side of this silicon oxide film 6. The polycrystalline silicon 7 is provided by means of the CVD method under low pressure for the purpose of being uniformly filled within the groove 5. The entire inside groove 5 can also be filled by means of SiO.sub.2 (nondoped, or the so-called BPSG or the like) which is doped with phosphorus (P)/boron (B) by means of the CVD method under low pressure.
An element, for example, a bipolar element, a MOS element, or a resistor element is formed in the element forming region surrounded by this trench-like element separating structure (in the figure, a MOS transistor is exemplified).
However, in the above-mentioned trench-like element separating structure used in the past, there are the following problems.
In particular, in the case of the SOI structure, because each element forming region of the substrate surface section 3 is separated like a floating island by means of the silicon oxide layer 4 and the silicon oxide film 6, there was the problem that there was no protection from the metallic contaminants and the like that intruded into each element forming region, and these metallic components caused the element characteristics to deteriorate. For example, the metallic contaminants intruded within the gate oxide film of the MOS transistor, and the voltage resistance of the gate oxide film decreased.
Thus, the purpose of this invention is to offer a semiconductor device and its manufacturing method having a trench-like element separating structure in which the metallic contaminants and the like that have intruded into each element forming region can be effectively collected, and the deterioration of the element characteristics due to these metallic contaminants can be reduced.